1. Field of the Invention
The present invention relates to a rubber thread to be used for a wound golf ball and a method of manufacturing the rubber thread.
2. Description of the Related Art
A golf ball to be used for a play in a golf course is roughly divided into a wound golf ball having a core on which a rubber thread is wound and a solid golf ball (two pieces golf ball, three pieces golf ball and the like) having a core formed of a solid rubber. In general, the wound golf ball is excellent in a hit feeling and control performance and the solid golf ball is excellent in flight performance and durability. The wound golf ball has been used for a long time. For a certain period, almost all first-class golf balls were represented by the wound golf balls. The solid golf ball developed later can be manufactured easily at a low cost. In recent years, therefore, more solid golf balls than the wound golf balls have been put on the market. Under the circumstances, a professional golf player and an advanced amateur golf player tend to prefer the wound golf ball excellent in the hit feeling and the control performance.
As a method of manufacturing a rubber thread for the wound golf ball, a so-called latex blend method has been used for a long time. In the latex blend method, first of all, a crosslinking agent, an additive and the like are mixed into a rubber latex. Next, a conveyer belt having a surface to which a liquid coagulant is stuck is caused to pass through a latex composition. Consequently, the latex composition is stuck to the conveyer belt and coagulates. Thus, a rubber sheet is obtained. The rubber sheet is cut to have a predetermined width after the crosslinking. Consequently, a rubber thread is obtained.
Since the rubber thread obtained by the latex blend method has a uniform thickness, there is an advantage that the rubber thread is broken with difficulty when it is to be wound with a high draw ratio (usually, 600% to 1000%). Moreover, since the rubber thread obtained by the latex blend method has a low modulus, it is sufficiently stretched with a comparatively small tension. Therefore, the rubber thread obtained by the latex blend method can be wound as a core with a high density. A golf ball having the core has great resilience performance.
However, a liquid rubber latex is used in the latex blend method. Therefore, a working efficiency is poor and a working environment is not preferable. In addition, the rubber latex is expensive. Consequently, the cost of a material for the golf ball is increased.
As another rubber thread manufacturing method, Japanese Laid-Open Patent Publication No. 56-34736 (1981) has disclosed a dry blend method (which does not use the rubber latex). In the dry blend method, a solid rubber, a crosslinking agent, an additive and the like are kneaded by means of a kneading machine (a kneader, a Banbury mixer or the like) to obtain a rubber composition which is to be sheet-shaped by means of a calender or the like. The sheet is cut so that a rubber thread is obtained. In the dry blend method, a compounding agent is easily mixed by the kneading machine and a process is therefore simple. Accordingly, a high working efficiency can be obtained. Moreover, a working environment is also preferable.
However, a rubber thread obtained by the dry blend method has a nonuniform thickness and it has a rough surface. Therefore, the rubber thread has such a drawback that breakage (so-called thread cutting) is apt to be caused during winding. If the rubber thread is thick, the thread cutting is caused with difficulty. However, the thick rubber thread is stretched insufficiently. For this reason, a core has a low rubber thread density. A rubber thread blending an activator such as zinc oxide in a large amount causes the thread cutting with difficulty. In this case, a modulus of the rubber thread is increased and a density of the rubber thread in a core is reduced. A golf ball using a core with a low density of the rubber thread has a low hardness and a low resilience coefficient.
Japanese Laid-Open Patent Publication No. 11-4913 (1999) has disclosed a rubber thread using a natural rubber subjected to a protein removing treatment. The rubber thread is obtained by the dry blend method and causes the thread cutting with difficulty. In addition, the rubber thread contributes to an increase in resilience of the golf ball. However, the rubber thread is obtained through the protein removing step as described above. Therefore, a manufacturing process is complicated.
There is also supposed means for extruding a rubber composition obtained by the dry blend method like a sheet, carrying out a roller processing on the sheet-shaped rubber composition to reduce a thickness (Japanese Patent Application No. 11-344823(1999)). By the roller processing, the thickness of the rubber thread is made uniform and the thread cutting can be controlled to some extent. However, the surface roughness of the rubber thread cannot be reduced sufficiently by the roller processing. Accordingly, the thread cutting is generated more easily than that of a rubber thread obtained by the latex blend method.
Under the actual circumstances, thus, there has not been obtained a rubber thread which has a great workability during manufacture and causes thread cutting with difficulty.
In consideration of such a problem, it is an object of the present invention to provide a rubber thread for a golf ball which is obtained by a dry blend method but is stretched easily and is broken with difficulty during the stretch.
In order to achieve the object, the present invention provides a method of manufacturing a rubber thread for a golf ball comprising the following steps (a) and (b):
(a) feeding a rubber composition between a heat roller and a belt pressed against the heat roller and pressurizing the rubber composition by means of the heat roller and the belt, thereby obtaining a crosslinked sheet material; and
(b) cutting the crosslinked sheet material to obtain the rubber thread.
In the manufacturing method, the rubber composition is crosslinked while being pressed by the heat roller and the belt. Therefore, the rubber thread thus obtained has a uniform thickness and a small surface roughness. Accordingly, when the rubber thread is to be wound, thread cutting is caused with difficulty. By the manufacturing method, therefore, the thickness of the rubber thread can be reduced. Since the rubber thread having a small thickness can be stretched easily, a density of the rubber thread in a core can be increased. In the manufacturing method, moreover, a rubber latex is not used. Consequently, a great workability and a preferable working environment can be obtained.
It is preferable that a pressure to be applied to the rubber composition at a crosslinking step should be 0.03 MPa to 1 MPa. Accordingly, a small surface roughness of the rubber thread is compatible with a low modulus.
Preferably, the sheet-shaped rubber composition extruded from an extruder is supplied at the crosslinking step. Consequently, the thickness of the rubber thread can be made more uniform.
It is preferable that a ten-point mean roughness Rz of the rubber thread is 40 xcexcm or less. Consequently, the thread cutting can be prevented more greatly. Preferably, the rubber thread has a thickness of 0.35 mm to 0.6 mm. Similarly, the thread cutting can be prevented more greatly.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.